A cracked heat exchanger turns a furnace repair into a safety question, because the crack can let combustion gases, including carbon monoxide, into the air you breathe. This guide covers the warning signs, how a technician should prove a crack before anyone talks about replacement, and what each fix costs.
Quick starting point: If a carbon monoxide alarm is sounding, don’t troubleshoot. Get everyone outside, then call 911 or Piedmont Natural Gas at (800) 752-7504 from there. The full steps are in the safety section of our furnace guide.
What Is a Cracked Heat Exchanger, and Is It Dangerous?
A heat exchanger is the sealed metal chamber inside a gas furnace that the burner flame heats while the blower pushes house air across the outside. Its wall is all that keeps flue gases on one side and the air going to your rooms on the other. A crack breaks that seal. Depending on how the furnace is burning, the gap can let combustion gases, including carbon monoxide, into the airstream, or let blower air push into the burner area and disturb the flame. Carbon monoxide has no color or smell, and CO poisoning feels like the flu: headache, dizziness, weakness, nausea and confusion. That is why a confirmed crack means the furnace stays off until it’s fixed or replaced.
Most cracks come from heat stress. The metal expands every time the burners light and contracts when they shut off, and after enough seasons it can split at a seam or bend. A furnace that runs hot for years from a clogged filter, closed vents or undersized ductwork wears out faster.
What Are the Signs of a Cracked Heat Exchanger?
Most cracked heat exchangers give no sign a homeowner can see, which is why the check belongs in a yearly furnace inspection. When there are signs, these are the ones to take seriously:
- A carbon monoxide alarm going off while the furnace is running.
- Flames that change when the blower starts. Healthy flames stay steady and mostly blue. Flames that jump, roll or turn lazy and yellow when the blower kicks on suggest air is moving through the heat exchanger.
- Soot on the burners, around the burner opening or near the registers.
- The furnace shutting down on a safety switch, especially again after a reset.
- Headaches or flu-like symptoms at home that ease when everyone is away.
Rust, an old furnace or a noisy inducer motor are reasons to look closer, not proof of a crack. Any of the signs above is a reason to shut the furnace off and have it tested.
How Does a Technician Confirm a Cracked Heat Exchanger?
A crack should be confirmed with evidence you can see. No single test catches every crack, so a careful technician combines a few checks, and you can ask to see what each one found:
| Test | What the technician does | What to ask to see |
|---|---|---|
| Flame check | Watches the burner flames as the blower starts | A short video if the flames move |
| Combustion analysis | Measures carbon monoxide and oxygen in the flue gas, before and after the blower starts | The readings, and how they changed |
| Visual or camera inspection | Looks at the metal through the burner openings, often with an inspection camera | A photo of the crack itself |
| CO in the living space | Measures carbon monoxide at the registers and in the rooms | The reading and where it was taken |
A photo of the crack or the written readings is a fair thing to ask for before you approve a replacement. If you want a second opinion on a diagnosis this serious, get one. A real crack will still be there.
Can a Cracked Heat Exchanger Be Repaired?
No. A cracked heat exchanger isn’t welded or patched on a home furnace. The fix is a new heat exchanger or a new furnace, and until then the furnace should stay off. A patch can’t hold up to the heating and cooling the metal goes through every cycle, and no manufacturer warranty covers one. If a crack is confirmed mid-winter, use space heaters only as the manufacturer directs, plugged straight into a wall outlet and away from anything that can burn.
How Much Does It Cost to Fix a Cracked Heat Exchanger?
The cost depends on whether the heat exchanger is still under warranty. Furnace makers warrant it far longer than the rest of the furnace: American Standard, one of the brands we install, covers it for 20 years, and some brands cover it for life on registered units. The warranty pays for the part, not the labor, and swapping a heat exchanger means taking most of the furnace apart. National cost guides put the job at about $1,000 to $3,000 out of warranty, mostly labor.
| Option | Typical cost | When it makes sense |
|---|---|---|
| New heat exchanger, part under warranty | Labor only, often $650 to $2,150 (national) | Furnace under about 10 years old, otherwise healthy |
| New heat exchanger, out of warranty | About $1,000 to $3,000 (national) | Rarely; compare it to a new furnace first |
| New system | $8,500 to $22,000 (Andersen’s published tiers) | Furnace past about 15 years, or other parts failing |
Andersen’s tiers are for a full heating and cooling system. A furnace-only replacement is quoted at the in-home visit, and our furnace and heat pump replacement guide covers what moves the price.
Should You Replace the Heat Exchanger or the Whole Furnace?
Replace the heat exchanger when the furnace is young, the part is under warranty and nothing else is wearing out. Replace the furnace when it’s past about 15 years, out of warranty, or the heat exchanger is one of several problems at once. A useful test: if the repair, labor included, comes to half the price of a new furnace or more, the new furnace is usually the better spend, because the rest of an older furnace is close to the end too.
Several problems at once is common. For one longtime Andersen customer, a semi-annual maintenance visit turned up both: “The furnace was not usable due to the faulty heat exchanger and the AC compressor was leaking Freon.” They replaced the system once instead of paying for two big repairs in a row.
Where Should Carbon Monoxide Alarms Go, and How Long Do They Last?
Put a carbon monoxide alarm on every level of the house and outside each sleeping area, with a battery or battery backup so it works in a power outage. Interconnected alarms are best: when one sounds, they all sound, so an alarm near an attic furnace wakes the bedrooms too. Test them monthly and check the batteries when the clocks change.
CO alarms wear out. The CDC says to replace a CO alarm by the manufacturer’s instructions or every five years. Most print a replace-by date on the back, and many chirp at end of life. An expired alarm may not sound when it should.
A CO alarm is the last line of defense, not an inspection. Alarms sound when carbon monoxide builds to a harmful level, so a slow, low leak can go unnoticed. The furnace still needs a yearly check.
How Can You Keep a Heat Exchanger From Cracking?
The best protection is steady airflow and a yearly inspection. Check the filter monthly in heating season, keep supply registers open and return grilles clear, and have the furnace serviced every fall. Andersen’s fall maintenance visit includes inspecting the heat exchanger and flue, checking gas pressure and cleaning the burners, which catches overheating and early damage while the fix is still a choice. Members of our Preferred Maintenance Agreement get two visits a year, pay no diagnostic fee and take 15% off any repair. Our fall HVAC maintenance checklist covers the rest of a fall visit.
Frequently Asked Questions
Can I run my furnace with a cracked heat exchanger?
Andersen Heating & Cooling recommends against it. Once a crack is confirmed, turn the furnace off and leave it off until the heat exchanger or the furnace is replaced. Running it risks carbon monoxide reaching the living space, and a crack only grows with every heating cycle.
Will a carbon monoxide alarm detect a cracked heat exchanger?
Sometimes, but not reliably. A CO alarm can catch a dangerous leak, but it won’t catch a crack early. Keep working alarms on every level, and have the heat exchanger itself inspected every fall.
Should I get a second opinion on a cracked heat exchanger?
Andersen Heating & Cooling considers a second opinion reasonable on any diagnosis that leads to a furnace replacement. Ask for a photo of the crack or the combustion readings first, and take them with you to the second contractor.
Get Your Furnace Checked in Charlotte
If your furnace keeps shutting down, shows any of the signs above, or a technician has told you the heat exchanger is cracked, call Andersen Heating & Cooling at (704) 596-2525 or request service online. We book furnace repairs seven days a week from 7:30 a.m. to midnight, with no weekend or after-hours upcharge, and you get the price before any work starts. See our furnace and heating repair page for what a repair visit includes. Andersen holds North Carolina HVAC license L.34538 and has served Charlotte homeowners since 1984.